1. Field of the Invention
The present invention relates to an optical projection system for projecting an enlarged image of an image displayed on an image display element onto a projection plane such as a screen as well as to an image projector incorporating such an optical projection system.
2. Description of the Related Art
An image projector comprises a lamp as a light source, an image display element such as a light valve including a DMD (Digital Micromirror Device) or a crystal liquid display panel, and an optical projection system. The display of the image display element is illuminated with a light beam from the lamp and an enlarged image of an image on the display surface is projected onto a screen as a projection plane with the optical projection system. DMD as an image display element includes a large number of micromirrors each of which angle is electrically controllable in a range of +12 to −12 degrees. For example, an incidence angle of light on the DVD is set such that reflected light by a single micromirror at an angle of −12 degree is incident on the optical projection system but reflected by the micromirror at an angle of +12 degrees is not incident on the optical projection system. Thereby, it is possible to form a digital image on the display of the DMD and project the image on the screen via the optical projection system by controlling the tilt angle of each micromirror of the DMD.
In recent years an ultra-short throw projector has been widespread, which can project an image on a screen placed in a very short distance. Such an ultra-short throw projector aims to avoid projected light from entering the eyes of a presenter or a speaker present close to the screen and avoid placing a projector around audience of a presentation to prevent the audience from suffering discharged air and noise from the projector.
A liquid crystal projector is a well-known image projector and has been advanced in terms of high resolution and improved brightness along with development of a high-efficiency lamp and price reduction. Also, a light-weight compact image projector adapting DMD is available and widely used not only in offices and schools but also at home.
Furthermore, there have been demands for downsizing a projector body and shortening a projection distance. In view of this, Japanese Laid-open Patent Application Publication No. 2011-253023 (Reference 1) and No. 2011-242606 (Reference 2) disclose an optical projection system and an image projector incorporating a reflection-type image forming optical system using a free form surface, for instance. Reference 1 describes an optical projection system including a free-form surface lens to correct a tilted projection. Reference 2 describes an optical projection system incorporating a free-form surface mirror. An optical system using a free-form surface mirror is effective in terms of downsizing a projector and shortening a projection distance. However, distortion and field curvature are corrected only with the free-form surface mirror, which brings enlargement of the size of the mirror. Along with an increase in the size, the free-form surface mirror becomes more extendable by a temperature change, and distortion or defocus by a temperature change is more likely to occur. The above References 1 and 2 fail to address this problem. There is an optical projection system comprising a rotationally asymmetric free-form surface lens, aiming to correct distortion and decrease projection distance. However, related art has failed to deal with a desired lens shape and a contrivance for reducing a change in distortion by a temperature change and internal birefringence of a plastic lens.